This invention relates to the pretreatment of natural gas prior to liquefaction and, more particularly, to the reduction of the carbon dioxide (CO.sub.2) content of natural gas.
There are probably over 60 LNG (liquified natural gas) plants in operation in the United States which receive natural gas at elevated pressure from transmission pipelines and liquefy it for storage. The stored LNG is used in periods of peak demand in a gas distribution system when the transmission pipeline alone cannot satisfy the peak demand.
The natural gas delivered to LNG plants contains moisture and CO.sub.2. To date, pipeline operators have been able to deliver natural gas containing not more than about 1.0% by volume of CO.sub.2. However, in recent years, the wells have been producing natural gas with a slowly increasing content of CO.sub.2. To liquefy natural gas, it is necessary to first eliminate moisture and CO.sub.2 because both of these impurities would solidify at the temperatures required to liquefy natural gas and the solids thus formed would plug the equipment and render the LNG plant inoperative.
All LNG plants in this country use one of two basic systems for eliminating moisture and not more than about 1.0% by volume of CO.sub.2. The system of the great majority of LNG plants is hereinafter referred to as system A while system B will refer to that of the other LNG plants.
System A comprises a group of vessels filled with molecular sieves adapted to adsorb the moisture of natural gas passed therethrough, and a second group of vessels filled with molecular sieves adapted to adsorb the CO.sub.2 of natural gas passed therethrough.
System B comprises means for injecting methanol into the natural gas, for chilling the gas to condense all of the moisture and methanol, and for removing the condensed moisture and methanol from the dry natural gas which is then passed through a CO.sub.2 absorption unit involving the use of methanol to remove CO.sub.2 from the dry natural gas and the use of a stripping gas to eliminate absorbed CO.sub.2 from the methanol so that it can be recycled for the absorption and removal of CO.sub.2 from the dry natural gas.
Existing systems A and B have been designed and built to dry natural gas and to eliminate as much as about 1.0% by volume of CO.sub.2. The dual facts that systems A and B are inadequate for natural gas containing about 1.5% by volume of CO.sub.2 or more and that natural gas from pipelines can no longer be expected to have a maximum CO.sub.2 content of 1.0% by volume have created an urgent search by LNG plant operators for an economically feasible plan for averting the prospective disastrous shut-down of their LNG plants because of increased CO.sub.2 content of natural gas available from transmission pipelines.
A principal object of this invention is to provide an economically feasible pretreatment of natural gas containing in excess of 1.0% by volume of CO.sub.2 prior to liquefaction.
Another important object is to integrate equipment of existing LNG plants with additional equipment required by the new pretreatment of natural gas and thus minimize the purchase of additional equipment.
A further object is to provide a flexible pretreatment that is capable of giving adequate removal of CO.sub.2 from natural gas in spite of foreseeable variations in the CO.sub.2 content of natural gas delivered to LNG plants.
These and other objects and advantages of the invention will be evident from the description which follows.